This drink was very likely created by Galliano, here is an early publication of this drink:
Show: the magazine of the performing arts: Volume 4, Part 1 in 1964
"GOLDEN CADILLAC I oz. Galliano I oz. White Creme de Cacao I oz. Cream Place in blender with small quantity of crushed ice. Use low speed for short time until creamy. Pour into champagne glass."
Galliano ran an advertising campaign for this drink from 1964 - 1967 and claim the drink as their own, which it likely is. This drink belongs to a family of drinks known as the galliano family, which also falls under the Galliano era. This drinks name is derived from the smash Elvis hit "Solid Gold Cadillac" as seen here:
Billboard - Nov 3, 1956 - Page 38
"ASCAP Solid Gold Cadillac (R) (F)— Columbia"
This drink is commonly believed to be served in a cocktail glass without any ice, when it is originally served frozen or blended. There is no specific location of origin associated with this drink. This drink dates between 1956 - 1957 and contained at least as of then:
Blend these ingredients:
White creme de cacao
pour contents into a champagne glass.